lookup or some other type of similar function
 

I have a workbook set up with two pages, one that stores the data (sheet1)
and one that summarizes data on a monthly basis (sheet2).
Column A of sheet1 holds the name of the month, column b stores the total
sales for that particular month.
On the summary page (sheet2) I currently have a vlookup function where i
update the name of the month each month (say, in cell A1, if i am reporting
for September, i will put "September" in cell A1) and cell C5 contains a
vlookup formula that will extract the total sales for the month of september.
If i haven't lost you yet, i am trying to add a similar function for Year to
Date totals. So, if i want a simiilar function on sheet2, where the first
month (typically january) will remain static but the ending month will be
dynamic (changing to the current reporting period each month)...

....how would i ask the program to lookup all of the values in column B on
sheet1 from the static month through the dynamic month?

Thanks.